1. Field of the Invention
The invention relates to impact dot heads for impact dot matrix printers. More particularly, the invention is directed to an impact dot head that can prevent breakage of wires by arranging the wires so as to be resiliently oscillatable in a printing digit direction.
2. Related Art
An exemplary conventional impact dot head is shown in FIGS. 54 and 55. As shown in FIG. 54, an impact dot head is designed to form a dot by projecting a wire 8 and applying impact onto a printing sheet 19 on a platen 12 through an ink ribbon 21 while moving in a direction parallel with the platen 12 (the direction perpendicular to the surface of the sheet in FIG. 54). In order to form a dot at a predetermined position on the printing sheet 19, each wire 8 is supported while arranged so as to be oscillatable by intermediate guides 7 and a front end guide 6.
A method of mounting the front end guide 6 has been proposed in Japanese Patent Unexamined Publication No. 530/1990. This impact dot head is provided as securing the front end guide 6 by inserting the front end guide 6 and the intermediate guides 7 into a nose guide 18 as shown in FIG. 55. Further, the nose guide 18 is secured by being inserted into a nose 1 made of a metal. The nose 1 has an oval window portion 30 that can accommodate the nose guide 18 with the front end guide 6 being secured thereto. The front end guide 6 is positioned by front end guide grooves 23 of the nose guide 18 in a direction perpendicular to the printing sheet and by the oval window portion 30 of the nose 1 in a direction parallel with the printing sheet.
The thus arranged impact dot head 400 is, as shown in FIGS. 56 and 57, fixed on a carriage 401 by fitting projecting portions 403 arranged on the carriage 401 into mounting holes 402 arranged on the nose 1 and screwing them.
In a conventional impact dot printer, when printing is started from a position outside a printing sheet due to erroneous setting of a printing area, the wire 8 gets broken as the impact dot head passes the end of the printing sheet while performing the printing operation. Amid diversification of printing paper, printing tends to be performed on thick paper or thick multi-sheet printing paper such as a duplicating slip, and the printing on such thick paper has often caused the trouble of wire breakage at the end of the printing sheet, which is a problem to be overcome.
How the wire gets broken when the impact dot head passes the end of the printing sheet while performing the printing operation will be described below.
FIG. 58 is a cutaway plan view of FIG. 54 as viewed from a position F. As shown in FIG. 58, the impact dot head starts printing at a position outside the printing sheet 19. When the head passes the end of the printing sheet 19 with the wire 8 driven to project while moving from left to right as viewed in FIG. 58, the wire 8 collides against the end of the printing sheet 19 and receives a force A. As a result, the wire 8 is flexed and receives a reaction B from a guide hole 32 arranged on the front end guide 6. If it is supposed that the frictional coefficient between the wire 8 and the ink ribbon 21 is .alpha. and the frictional coefficient between the wire 8 and the front end guide 6 is .beta., then frictional forces C and D act on the wire 8 in a direction opposite to a direction in which the wire 8 returns (the direction indicated by an arrow X). These frictional forces C (=.alpha.A) and D (=.beta.B) impede the wire 8 from returning. Therefore, the impact dot head continuously moves from left to right as viewed in FIG. 58 with the wire 8 having projected, which causes the wire 8 to be further flexed. As a result, when the bending stress of the wire 8 exceeds the allowable stress, the wire 8 gets broken, which makes normal printing impossible thereafter because there are missing dots.
While a case where the printing operation is performed from the position outside the left end of the printing sheet to right has been described, the same holds true for a case where printing is done from a position outside the right end of the printing sheet to left.
As a method of avoiding wire breakage applied to the conventional impact dot head, it is conceivable to significantly increase the returning force of a return spring 15 shown in FIG. 54 so as to exceed the sum of the frictional forces C and D. However, if the returning force of the return spring 15 is increased so much, a printing force large enough to overcome such large returning force must be obtained, which entails an extremely large drive current. That is, if the returning force of the return spring 15 is increased to a significant degree, the power capacity must be increased. In addition, an increase in the drive current results in building up heat in the head.
Therefore, the problem of wire breakage caused at the time the impact dot head passes the end of the printing sheet during printing has been substantially unavoidable.